Your friend's assertion that action and reaction force pairs cancel each other out and thus do not change motion is a misunderstanding of Newton's third law of motion and the nature of forces acting on objects. Let's clarify this with an everyday example and contrast action-reaction force pairs with balanced forces.
Newton's Third Law of Motion
Newton's third law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on a second object, the second object exerts a force of equal magnitude in the opposite direction back on the first object. Importantly, these forces act on different objects and do not cancel each other out in terms of changing the motion of either object.
Everyday Example: Walking
Consider the act of walking. When you place your foot on the ground and push down against it, you are applying a force (the action force). In response, the ground exerts an equal and opposite force (the reaction force) on your foot, pushing you upward and forward. Here, the action-reaction forces are:
- Action force: Your foot pushing down on the ground.
- Reaction force: The ground pushing up on your foot.
Although these forces are equal in magnitude and opposite in direction, they do not cancel each other out because they act on different objects (your foot and the ground). The net result is that you move forward, which illustrates how action and reaction forces can indeed change the motion of an object—in this case, your whole body.
Contrasting Action-Reaction Force Pairs and Balanced Forces
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Action-Reaction Force Pairs:
- Act on different objects.
- Do not cancel each other out as they cause different objects to move.
- Example: The force of your foot pushing down on the ground (action) and the ground pushing up on your foot (reaction).
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Balanced Forces:
- Act on the same object.
- Result in a net force of zero, leading to no change in motion (the object remains at rest or moves at a constant velocity).
- Example: If you are pushing a stationary box with a force of 5 N to the right while friction exerts a force of 5 N to the left, the forces are balanced, and the box does not move.
In summary, while action-reaction force pairs arise from interactions between different objects, balanced forces are involved when multiple forces act on the same object and combine to zero net force. Understanding this distinction shows why action-reaction pairs are crucial in changing an object's motion, as seen in the walking example.
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